Linear motors are commonly used in semiconductor assembly equipment as a fast and accurate means to drive mechanical parts as they can provide relatively higher system performance as compared to rotary servo motors that are coupled with rotary to linear conversion mechanisms. This is due to the elimination of added inertia, friction, compliance and backlash that typical rotary-to-linear mechanisms have. High force density, excellent response and high reliability are some advantages of brushless linear motors.
A linear motor comprises permanent magnets that are arranged to form a magnetic field and a coil assembly that is disposed within the magnetic field which is configured for carrying a current. Either the set of permanent magnets or the coil assembly is typically kept stationary while the other component is configured to be movable relative to it in order to drive a payload.
A main source of heat generation in a linear motor occurs in its force-producing coil assembly, and this often places a heat generator in close proximity to the payload. Furthermore, in conventional linear motors, the gaps allowed between the coil assembly and the permanent magnets are usually very small to maximize efficiency, resulting in inefficient heat dissipation between the respective surfaces of the coil assembly and the permanent magnets to the environment. In high accuracy applications, this will be a concern since dimensional stability is affected by changing ambient thermal conditions. In addition to the moving payload, the system's own components such as guides or feedback may react negatively to elevated temperatures and lead to safety issues or even failure of the linear motor.
It is thus necessary to implement an apparatus to prevent such adverse effects. Providing a thermal insulator between the coil assembly and the rest of the system may be one approach, but this might significantly de-rate the motor due to effective loss of its heat sink. Therefore, many linear motors are often offered with internal air cooling.
One type of air cooling apparatus that has been implemented in the prior art to cool linear motors is described in U.S. Pat. No. 5,834,862 entitled “Linear Motor Cooling System”. A cooling system is described therein for a closely coupled linear motor including a moving coil mounted for movement on a stator core. A nozzle comprising a base member and a cover plate is mounted on one end of the moving coil for producing a pair of high velocity sheets of air which are directed horizontally over the surface of the exposed turns so as to rapidly cool and stabilize the temperature of the coils.
However, these high velocity sheets of air are generated in directions that are perpendicular to the gaps between the coil frame and the magnet assembly, and not directly into the gaps. The sheets of air are instead made to traverse curvatures in the coil frame before entering the gaps. This approach of initially generating the air-flow perpendicular to the gaps is likely to result in the loss of cooling air into the atmosphere while the cooling air traverses the curvature of the frame. This is especially so when the radius of curvature of the frame is small, and would in turn reduce the amount of cooling air that is available to enter the gaps and pass over the heated coils. The cooling effect would thus be less efficient. It would be advantageous to generate cooling air directly into the gaps between the coils and the permanent magnets, while at the same time amplify the cooling air flow.
It would be appreciated that conventional cooling apparatus such as the aforesaid utilize a large volume of compressed air to generate a cooling air flow. It is desirable to reduce the volume of compressed air required for cooling purposes. Furthermore, the cooling air flow so generated may leak into the surroundings and inadvertently contaminate a clean production environment. More efficient use of cooling air and maintaining a cleaner production environment may be achieved by taking measures to control and to limit the air flow as far as possible only across surfaces that need to be cooled and not into the production area.